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Diadema sea urchins
dead sea urchin
Until the late 1970s, Caribbean reefs were occupied primarily by reef-building corals that exhibited a pronounced zonation pattern. Elkhorn coral (Acropora palmata) formed large, monospecific stands in the reef crest and shallow fore reef (0-5 m depth); staghorn coral (A. cervicornis) occurred from 5-25 m depth; massive star corals (Montastraea annularis complex) were common throughout the fore reef (5-30 m depth) and in back reef and lagoonal areas; and plating corals (especially Agaricia) inhabited deeper reefs.
Beginning in the 1970s, reefs experienced a step-wise decline. Outbreaks of white-band disease spread throughout the Caribbean, devastating populations of staghorn and elkhorn coral. The long-spined black sea urchin, Diadema antillarum, experienced a mass die-off (1982-1983) leading to a proliferation of fleshy seaweeds which began to overgrow corals and limit the potential for successful settlement and survival of coral planula. Macroalgae was much more pronounced in locations with heavy fishing pressure on herbivorous parrotfishes.
During the 1990s, the most important massive framework corals, Montastraea annularis (species complex), begun to exhibit a conspicuous trend of decline from coral diseases. White plague and yellow band disease, often linked to mass coral bleaching events, caused rapid loss of these corals. White plague can spread 1-10 cm per day, killing a 100-500 year old coral within weeks to months. Coral bleaching concurrently devastated reefs in 2005, 2009 and 2010.
Populations of other species, especially top predators such as sharks and groupers, are much less common today. As fishing pressure continues to increase, fishermen target new species, gradually fishing down the food chain. Thirty-five years ago, parrotfish were harvested in only a few countries. Today, they are often the most common species found in fish markets.
The consequences of removing too many fish has a trickle down effect on reef health. Removal of predatory groupers can lead to increases in small omnivores and herbivores such as damselfish. Certain damselfish (e.g., three-spot damselfish) become a nuisance as they promote the growth of algal lawns within their territories, and this algae can overgrow and kill corals. Overharvesting of parrotfish can also result in a proliferation of macroalgae, especially if there are added nutrients and other herbivores are rare.
Over a 35 year period, live coral cover has declined from 50-70% to 2-10%. It may take decades to return to levels seen in the 1970s. Yet, there are places where management measures have been effective, and coral cover has begun to rebound.
Can Caribbean reefs return to their former glory?
Images (left to right)
Montastraea faveolata :
- Yellow-band disease
- Damselfish algal lawn
NB: the three species of the Montastraea annularis complex described here have been reclassified into the genus Orbicella
healthy elkhorn coral stand